Maps and the discipline of mapping, developed by two centuries of practice with paper and sharp pencil, are integral to geology as we know it. As our spatial data become digital, we are obviously concerned that we do not lose the benefits that paper and pencil bring us. Most discussion to date on digitization has been over how to put paper maps into the computer. But we should think further: Digital spatial databases aren't just a faster and cheaper way to produce and deliver geologic maps. As our spatial data become digital, we will change and improve the way we do geology.

In this essay I discuss what a geologic map is, how geologic maps differ from other maps, and what a geologic spatial database is and how it differs from a map. My hope is to encourage clarity on these topics as we discuss standards for digital geologic spatial data. I then propose several years of explicit experimentation before national standards for digital geologic spatial data are adopted. As a contribution towards this discussion and experimentation, I conclude with some suggestions towards better databases.

My qualifications are limited. I have no formal training in cartography, GIS theory, or database design. I am a geologist with nearly two decades of experience mapping in the humbling terrane of northwest Washington (e.g. Tabor and others, 1994; Haugerud and others, 1994). I have the good fortune to have worked closely for much of this time with an excellent mapper and to have been exposed to many of the other wise men and women within the USGS. I have used Arc/Info for several years. A recent project in regional map compilation (Haugerud, 1997) has turned me into a consumer of digital geologic spatial data. And I have been a peripheral participant in two of the ongoing standards-generation efforts (Raines and others, 1997; Fitzgibbon and Wentworth, 1991).

What is a Geologic Map?

"There isn't a unique, correct, geologic map."
E.H. Brown to field camp students, 1974

"Plot a symbol, some symbol, to show each outcrop that you visit."
Eric Cheney to field camp students, circa 1985

"A geologic map is an expression of an hypothesis.
(Geologists who make blob maps have blobby hypotheses.)"
E-an Zen, 1986

"Plot as you plod, or you won't know where to go next."
Rowland Tabor (paraphrased), many times

A geologic map is (1) a record of observations located in space, (2) an expression of an hypothesis about Earth history, and (3) a tool for analysing Earth history. It is NOT a simple, or even an abstract, description of some part of the Earth.

The Map as Record of Observations

First of all, our maps are records of our observations. A bedding attitude here, a sample locality there. But many, even most, of the observations shown on a geologic map are not simple observed facts. How many times have you explained to an assistant why your notes don't mention the red color, or the extensive fracturing, or the (fill in the blank) of some outcrop? We do not simply record what can be seen. Our field "data" are already filtered through a large body of geologic theory. Sometimes our observations are no closer to being facts than the hypotheses we attempt to test with these observations. This is the major reason that advances in structural, stratigraphic, or petrologic theory commonly necessitate remapping of regions. The point is driven home when you discover that you and your colleague, with over five decades field experience between you, cannot agree on the orientation of bedding at the last outcrop.

The Map as Expression of an Hypothesis

Most geologic mappers see only a fraction of a map area, but possess powerful theoretical tools (Steno's Laws, expectations of facies patterns, and so on) that, with an Earth history inferred from these limited observations, allow extrapolation of geologic relations to the rest of the map sheet, even the areas that are covered. A geologic map is commonly more prediction than depiction. The map is an illustration of an inferred Earth history.

The amalgam of observation and inference on a map can be uneasy, as is discovered by the geologist who gets a helicopter ride to an inaccessible ridge only to discover that the foliation attitude plotted by a revered predecessor was wishful thinking. Unfortunately, paper usually does not allow the density and detail of symbolization that would be needed to fully distinguish between what we saw, what we think we saw, and what we think we would have seen if we could have seen.

The Map as Analytic Device

A geologic map is also an analytic device. This is true after it is made -- how many classes have we collectively taken, or taught, in which we were asked to unravel some aspect of Earth history from a map? But the making of a map is also an analytical tool. The mapper knows well the ritual: daily plotting of field observations on the office map; inking that which is well-known; erasing; coloring -- sometimes tentatively, sometimes with conviction -- of areas that have been mapped; erasing; poring over the uncompleted map, field sheets, and air photos; and long hours of discussion if one is fortunate enough to work with a colleague, always asking "What is the stratigraphy? What is the structure? Where do I traverse tomorrow to fill in the map, maximize outcrop, minimize travel costs, and best resolve the major unknowns in the geologic story?" Putting one's observations on mylar at 1:24,000 or 1:100,000 scale turns incomprehensible mountainsides into simple geologic relations that can be held in the hand -- doing so is a tool that allows a flea to see elephants. Putting a station in every square inch of the map enforces a completeness to one's analysis. And mapping all the unconsolidated deposits keeps one honest about what bedrock history can be known and what cannot. (And vice versa.)

How do Geologic Maps Differ from Other Maps?

Geologic maps differ from many other maps in important ways: (1) Our maps are commonly of entities that are observed with difficulty. Identification of a municipal boundary, or an interstate highway, is easier than the identification of many (most?) geologic map units. Uncertainties about classification of the mapped object are greater than with other types of maps. Misclassification errors are correspondingly more common. (2) Our maps are more sensitive to scale than other types of maps. The nature of a contact on a political map does not change with map scale: it is commonly defined, and observed, with much greater precision than it is plotted at a wide range of scales. Geologic objects are commonly defined and observed with a resolution that is near the intrinsic resolution of the map -- indeed, we choose our map scale or our observation method so that this is true, and we then use symbols to denote whether an object is located as well as or more poorly than can be depicted at that scale (e.g. continuous or dashed contacts). (3) Our maps are more complex. Whereas some cartographic dogma proclaims 'one theme, one map', geologic maps commonly display multiple themes (e.g. lithostratigraphy and topography) so that the user can observe the interplay between themes and use this interplay to make useful inferences. (4) Many geologic maps have text and correlation charts that describe rich and complex relations between the various units shown on the map. (5) A well-made geologic map has benefited from a great deal of thought about its symbolization. Colors, patterns, line-weights, symbol sizes, unit tags, and type fonts are all carefully chosen to reinforce explicitly recognized relations and to hint at others. (6) Because geologic maps are largely inference, their authorship is important.

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